You say that your review was due on the 31st Dec. Was this when your award was due to end, or a year before your award was due to end? PIP review forms are being sent out approximately 6 months before the end of an award.
If you award actually ended on the 31st Dec 2019 then you should ring the DWP to find out where your review form is.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems
